1. Field of the Invention
This invention relates to a method for forming a semiconductor crystal and a semiconductor crystal article obtained by said method, particularly to a method for forming a semiconductor single crystal or a semiconductor polycrystal controlled in grain size prepared by utilizing the difference in nucleation density of the deposited materials according to the kinds of the deposited surface materials, and a crystal article obtained by said method.
The present invention is applicable for formation of a semiconductor crystal such as a semiconductor single crystal or a semiconductor polycrystal to be used for electronic devices, optical devices, magnetic devices, piezoelectric devices or surface acoustic devices, etc., such as semiconductor integrated circuits, optical integrated circuits, magnetic circuits, etc.
2. Related Background Art
In the prior art, single crystal thin films to be used for semiconductor electronic devices or optical devices have been formed by epitaxial growth on a single crystal substrate. For example, it has been known that epitaxial growth of Si, Ge, GaAs, etc., can be done from liquid phase, gas phase or solid phase on Si single crystal substrate (silicon wafer), and it has been also known that epitaxial growth of a single crystal such as GaAS, GaAlAs, etc., occurs on a GaAs single crystal substrate. By use of the semiconductor thin film thus formed, semiconductor devices and integrated circuits, electroluminescent devices such as semiconductor lasers or LED have been prepared.
Also, researches and developments have been recently made abundantly about ultra-high speed transistors by use of two-dimensional electronic gas, ultra-lattice devices utilizing quantum well, etc., and what has made these possible is the high precision epitaxial technique such as MBE (molecular beam epitaxy) or MOCVD (organometallic chemical vapor deposition) by use of ultra-high vacuum.
In such epitaxial growth on a single crystal substrate, it is necessary to take matching in lattice constants and coefficient of thermal expansion between the single crystal material of the substrate and the epitaxial growth layer. For example, although it is possible to effect epitaxial growth of Si single crystal thin film on sapphire which is an insulating single crystal substrate, the crystal lattice defect at the interface due to deviation in lattice constant and diffusion of aluminum which is a component of sapphire to the epitaxial layer pose problems in application for electronic devices or circuits.
Thus, the method for forming a single crystal thin film of the prior art by epitaxial growth may be understood to be dependent greatly on its substrate material. Mathews et al have examined about combinations of the substrate material with epitaxial growth layer (EPITAXIAL GROWTH, Academic Press, New York, 1975, ed. by J. W. Mathews).
Also, the size of the substrate is presently about 6 inches for Si wafer, and enlargement of GaAs, sapphire substrate is further retarded. In addition, since the single crystal substrate is high in production cost, the cost per chip becomes higher.
Thus, for production of a single crystal layer capable of preparing a device of good quality according to the method of prior art, there has been involved the problem that the kinds of the substrate materials are limited to an extremely narrow scope.
On the other hand, researches and developments of three-dimensional integrated circuits to accomplish high integration and multi-function by laminating semiconductor devices in the normal line direction of the substrate have been abundantly made in recent years, and also reserches and developments of large area semiconductor devices such as solar batteries of switching transistors of liquid crystal picture elements, etc., in which devices are arranged in an array on a cheap glass are becoming more abundant from year to year.
What is common to both of these is that the technique for forming a semiconductor thin film on an amorphous insulating material and forming an electronic device such as transistor, etc., thereon is required. Among them, particularly the technique for forming a single crystal semiconductor of high quality on an amorphous insulating material has been desired.
Generally speaking, when a thin film is deposited on an amorphous insulating material substrate such as SiO.sub.2, etc., due to the defect of long distance order of the substrate material, the crystal structure of the deposited film becomes amorphous or polycrystalline. Here, the amorphous film refers to a state in which near distance order to the extent of the closest atoms is preserved, but no longer distance order exists, while the polycrystalline film refers to single cystal grains having no specific crystal direction gathered as separated at the grain boundaries.
For example, in the case of forming Si on SiO.sub.2 according to the CVD method, if the deposition temperature is about 600.degree. C. or lower, it becomes an amorphous silicon, while it becomes a polycrystalline silicon with grain sizes distributed between some hundred to some thousand .ANG. at a temperature higher than said temperature. However, the grain sizes and their distribution of polycrystalline silicon will be varied greatly depending on the formation method.
Further, by melting and solidifying an amorphous or polycrystalline film by an energy beam such as laser or rod-shaped heater, etc., a polycrystalline thin film with great grain sizes of some microns or millimeters have been obtained (Single Crystal silicon on non-single-crystal insulator, Journal of crystal Growth vol. 63, No. 3, Oct., 1983 edited by G.W. Gullen).
When a transistor is formed on the thus formed thin film of respective crystal structures and electron mobility is measured from its characteristics, mobility of about 0.1 cm.sup.2 /V.multidot.sec or less is obtained for amorphous silicon, mobility of 1 to 10 cm.sup.2 /V.multidot.sec for polycrystalline silicon having grain sizes of some hundred .ANG., and a mobility to the same extent as in the case of single crystalline silicon for polycrystalline silicon with great grain sizes by melting and solidification.
From these results, it can be understood that there is great difference in electrical properties between the device formed in the single crystal region within the crystal grains and the device formed as bridging across the grain boundary. In other words, the deposited film on the amorphous material obtained in the prior art becomes amorphous or polycrystalline structure having grain size distribution, and the device prepared thereon is greatly inferior in its performance as compared with the device prepared on the single crystal layer. For this reason, the uses are limited to simple switching devices, solar batteries, photoelectric converting devices, etc.
On the other hand, the method for forming a polycrystalline thin film with great grain sizes by melting and solidification had the problems that an enormous time is required due to scanning of amorphous or single crystal thin film with energy beam for every wafer to be poor in bulk productivity, and also that it is not suited for enlargement of area.
Further, in recent years, studies of diamond thin film growth are becoming popular. Diamond thin film, which is particularly broad in bandgap as 5.5 eV as the semiconductor, can be actuated at higher temperature (about 500.degree. C. or less) as compared with Si, Ge, GaAs, etc., which are semiconductor materials of the prior art. Also, the carrier mobility of both electrons and positive holes surpass that of Si (1800 cm.sup.2 /V.multidot.sec for electrons, 1600 cm.sup.2 /V.multidot.sec for positive holes), and thermal conductivity is also extremely high. For this reason, it has been expected to be promising for application in semiconductor devices of the great consumption power type with great heat generation quantity.
However, although there have been reports in the prior art about epitaxial growth of diamond thin film on a diamond subtrate by vapor phase growth (N. Fujimoto, T. Imai and A. Doi Pro. of Int. Couf. IPAT), there is no successful report about heteroepitaxial growth on a substrate other than diamond substrate.
Generally speaking, diamond nuclei are generated by utilizing excitation with microwave, using a hydrocarbon type gas such as CH.sub.4, etc., and by irradiation with hot filament or electron beam, but the nucleation density is generally low, whereby a continuous thin film can be obtained with difficulty. Even if a continuous thin film may be formed, it has a polycrystalline structure with great grain size distribution and can be difficulty applied for semiconductor device.
Also, as long as a diamond substrate is used, it is expensive as a matter of course, posing also a problem in enlargement of area. Thus, it is not suitable for practical application.
As described above, in the crystal growth method of the prior art and the crystal formed thereby, three-dimensional integration or enlargement of area could not be done with ease to be difficulty applied practically for devices, and crystals such as single crystals and polycrystals, etc., required for preparation of devices having excellent characteristics could not be formed easily and at low cost.